Addiction is a major health and social problem. The dopaminergic system is highly solicited by drugs of abuse, as an increased release of dopamine is a hallmark of the effect of drug intake. The dopamine D2 receptor (D2R) is a key component of the dopaminergic system. Indeed, in addition to its postsynaptic wide and abundant distribution in the central nervous system, it also functions as autoreceptor in the control of dopamine synthesis and release. D2Rs also modulate release of other neurotransmitters from postsynaptic neurons acting as a presynaptic heteroreceptor. D2Rs are composed in vivo by two isoforms, D2L and D2S, generated from the same gene by alternative splicing. The aim of this proposal is to demonstrate that the presynaptic autoreceptor/heterosynaptic control mediated by dopamine D2 receptors exerts a principal role in the behavioral and cellular responses to drugs of abuse. This hypothesis is based on preliminary studies conducted in our laboratory showing that absence of D2 receptors signaling in mice (D2R-/-) strongly alters the response to psychostimulants and other abused drugs. On the contrary, in D2L-/- animals that do not express the D2L isoform of the receptor with postsynaptic functions (i.e. behavioral and cellular pharmacological responses to agonists/antagonists), but maintain the expression of the presynaptic/heterosynaptic receptor functions (i.e. control of DA release, cocaine-induced effects on striatal GABA electrophysiological effects), D2S, the motor and rewarding effects of drugs are intact. Both D2L and D2S are co-expressed, as a result of the alternative splicing of the D2R gene, thereby preventing a clear definition of the D2R role by the use of classical knockouts and/or by pharmacological tools. Thus, to achieve this aim we have generated conditional D2R mouse mutants, in which the selective ablation of D2R sites from dopaminergic or striatal neurons can be performed. Behavioral, biochemical and immunohistochemical studies are proposed to analyze loss of D2R site-specific effects when animals are exposed to the psychostimulant cocaine. Using these different approaches we will be able to determine the impact of loss of D2 receptors at the behavioral and cellular levels and possibly to identify the molecular mechanisms underlying the altered response to drugs of abuse found in D2R deficient mice. This research may lead to novel therapeutic intervention to treat addiction. PUBLIC HEALTH RELEVANCE Addiction to drugs of abuse is a major social and health problem. To be able to design therapeutic interventions to treat addiction, a detailed knowledge of the molecular mechanisms activated by drugs in the brain is requested. Through the analysis of sophisticated animal models, questions on how the neurotransmitter dopamine modifies brain activity in response to the psychostimulant cocaine will be addressed. This may lead to important new therapeutic developments.